Re: [bpf-next PATCH] bpf: libbpf, support older style kprobe load

[Yonghong Song <yhs@fb.com>]

On 10/18/19 3:01 PM, John Fastabend wrote:
> Daniel Borkmann wrote:
>> On Fri, Oct 18, 2019 at 07:54:26AM -0700, John Fastabend wrote:
>>> Following ./Documentation/trace/kprobetrace.rst add support for loading
>>> kprobes programs on older kernels.
>>>
>>> Signed-off-by: John Fastabend <john.fastabend@gmail.com>
>>> ---
>>>   tools/lib/bpf/libbpf.c |   81 +++++++++++++++++++++++++++++++++++++++++++-----
>>>   1 file changed, 73 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index fcea6988f962..12b3105d112c 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>> @@ -5005,20 +5005,89 @@ static int determine_uprobe_retprobe_bit(void)
>>>   	return parse_uint_from_file(file, "config:%d\n");
>>>   }
>>>   
>>> +static int use_kprobe_debugfs(const char *name,
>>> +			      uint64_t offset, int pid, bool retprobe)
>>
>> offset & pid unused?
> 
> Well pid should be dropped and I'll add support for offset. I've not
> being using offset so missed it here.
> 
>>
>>> +{
>>> +	const char *file = "/sys/kernel/debug/tracing/kprobe_events";
>>> +	int fd = open(file, O_WRONLY | O_APPEND, 0);
>>> +	char buf[PATH_MAX];
>>> +	int err;
>>> +
>>> +	if (fd < 0) {
>>> +		pr_warning("failed open kprobe_events: %s\n",
>>> +			   strerror(errno));
>>> +		return -errno;
>>> +	}
>>> +
>>> +	snprintf(buf, sizeof(buf), "%c:kprobes/%s %s",
>>> +		 retprobe ? 'r' : 'p', name, name);
>>> +
>>> +	err = write(fd, buf, strlen(buf));
>>> +	close(fd);
>>> +	if (err < 0)
>>> +		return -errno;
>>> +	return 0;
>>> +}
>>> +
>>>   static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
>>>   				 uint64_t offset, int pid)
>>>   {
>>>   	struct perf_event_attr attr = {};
>>>   	char errmsg[STRERR_BUFSIZE];
>>> +	uint64_t config1 = 0;
>>>   	int type, pfd, err;
>>>   
>>>   	type = uprobe ? determine_uprobe_perf_type()
>>>   		      : determine_kprobe_perf_type();
>>>   	if (type < 0) {
>>> -		pr_warning("failed to determine %s perf type: %s\n",
>>> -			   uprobe ? "uprobe" : "kprobe",
>>> -			   libbpf_strerror_r(type, errmsg, sizeof(errmsg)));
>>> -		return type;
>>> +		if (uprobe) {
>>> +			pr_warning("failed to determine uprobe perf type %s: %s\n",
>>> +				   name,
>>> +				   libbpf_strerror_r(type,
>>> +						     errmsg, sizeof(errmsg)));
>>> +		} else {
>>> +			/* If we do not have an event_source/../kprobes then we
>>> +			 * can try to use kprobe-base event tracing, for details
>>> +			 * see ./Documentation/trace/kprobetrace.rst
>>> +			 */
>>> +			const char *file = "/sys/kernel/debug/tracing/events/kprobes/";
>>> +			char c[PATH_MAX];
>>> +			int fd, n;
>>> +
>>> +			snprintf(c, sizeof(c), "%s/%s/id", file, name);
>>> +
>>> +			err = use_kprobe_debugfs(name, offset, pid, retprobe);
>>> +			if (err)
>>> +				return err;
>>
>> Should we throw a pr_warning() here as well when bailing out?
> 
> Sure makes sense.
> 
>>
>>> +			type = PERF_TYPE_TRACEPOINT;
>>> +			fd = open(c, O_RDONLY, 0);
>>> +			if (fd < 0) {
>>> +				pr_warning("failed to open tracepoint %s: %s\n",
>>> +					   c, strerror(errno));
>>> +				return -errno;
>>> +			}
>>> +			n = read(fd, c, sizeof(c));
>>> +			close(fd);
>>> +			if (n < 0) {
>>> +				pr_warning("failed to read %s: %s\n",
>>> +					   c, strerror(errno));
>>> +				return -errno;
>>> +			}
>>> +			c[n] = '\0';
>>> +			config1 = strtol(c, NULL, 0);
>>> +			attr.size = sizeof(attr);
>>> +			attr.type = type;
>>> +			attr.config = config1;
>>> +			attr.sample_period = 1;
>>> +			attr.wakeup_events = 1;
>>
>> Is there a reason you set latter two whereas below they are not set,
>> does it not default to these?
> 
> We can drop this.

Agreed. attr.sample_period and attr.wakeup_events are used for perf.
Here we run bpf programs and return early, so these two values won't 
take effect.

> 
>>
>>> +		}
>>> +	} else {
>>> +		config1 = ptr_to_u64(name);
>>> +		attr.size = sizeof(attr);
>>> +		attr.type = type;
>>> +		attr.config1 = config1; /* kprobe_func or uprobe_path */
>>> +		attr.config2 = offset;  /* kprobe_addr or probe_offset */
>>>   	}
>>>   	if (retprobe) {
>>>   		int bit = uprobe ? determine_uprobe_retprobe_bit()
>>> @@ -5033,10 +5102,6 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
>>>   		}
>>>   		attr.config |= 1 << bit;
>>>   	}
>>
>> What happens in case of retprobe, don't you (unwantedly) bail out here
>> again (even through you've set up the retprobe earlier)?
> 
> Will fix as well. Looking to see how it passed on my box here but either
> way its cryptic so will clean up.
> 
>>
>>> -	attr.size = sizeof(attr);
>>> -	attr.type = type;
>>> -	attr.config1 = ptr_to_u64(name); /* kprobe_func or uprobe_path */
>>> -	attr.config2 = offset;		 /* kprobe_addr or probe_offset */
>>>   
>>>   	/* pid filter is meaningful only for uprobes */
>>>   	pfd = syscall(__NR_perf_event_open, &attr,
>>>
> 
>